The present invention relates to a surface acoustic wave (SAW) device and, more particularly to an SAW device in which noise caused by the pyroelectric effect of the piezoelectric substrate is suppressed.
In general, an SAW device such as an SAW filter has appropriately weighted interdigital transducers (IDTS) as input/output transducers on the surface of the piezoelectric substrate to perform conversion between an electrical signal and an SAW. In the conventional SAW device, an IDT pattern is merely formed of a conductive thin film on the piezoelectric substrate.
FIG. 3A shows an example of a conventional SAW device, and FIG. 3B shows the device of FIG. 3A along the line 3B--3B. As shown in FIGS. 3A and 3B, in the conventional SAW device, merely the pattern of IDTs 22 and 22' is directly formed of a conductive thin film on a piezoelectric substrate 21.
In the conventional SAW device shown in FIGS. 3A and 3B, when the ambient temperature changes, the surface of the piezoelectric substrate 21 is charged due to the pyroelectric effect of the piezoelectric substrate 21. When an electric field formed by the charges becomes equal to or more than the dielectric breakdown of the surrounding atmosphere of the piezoelectric substrate 21, the charges on the piezoelectric substrate 21 are discharged to the IDTs 22 and 22'.
If the charges are discharged to the ground-side IDT 22', the problem of noise does not occur in the SAW device. However, if the charges are discharged to the hot-side IDT 22, noise occurs in the SAW device.
In the latter case, the charges on the surface of the piezoelectric substrate 21 are not discharged unless they become equal to or more than the dielectric breakdown of the surrounding atmosphere of the piezoelectric substrate 21. Thus, the amount of charges removed at once becomes large to increase the noise level.
In particular, in an SAW device that uses lithium tantalate (LiTaO.sub.3) or lithium niobate (LiNbO.sub.3) as the material of the piezoelectric substrate 21, the piezoelectric substrate 21 has a strong pyroelectric effect, and the above drawback is remarkable.